A histogram is a type of graph. In photography, it is a way of plotting the exposure of a digital image. By looking at the graph, you can see whether a shot is too bright or too dark.

The histogram used by your camera is essentially the same as used by Photoshop – or whichever form of photo editing software you use. The brightness of each pixel in the image is given a value along a 256-step scale – where zero is the darkest, blackest value possible, and 255 is the brightest, whitest value. The 256 steps correspond to all the brightness values possible in a typical JPEG image, which uses an eight-bit scale, where the brightness of each of the three primary colours is given a binary value from 00000000 through to 11111111.

Using histograms is a much more accurate way of judging whether a picture needs a bit more, or less, exposure. It’s not just used for judging exposure: the shape and position of the graph can also tell you about the contrast of the lighting in the scene.

Below we’ve rounded up 6 ways to respond to common exposure problems when using histograms.

01 Under-exposed
The histogram is pushed to the left and fails to register any very bright areas on the graph. Retake the shot, dialling in a positive value of exposure compensation. Try a setting of +1, then check the new histogram.

02 Over-exposed
The histogram is stacking up at the extreme right-hand side, producing clipped highlights and burnt-out clouds. Retake the shot, dialling in a negative value of exposure compensation. Try a setting of -1.

03 High key
Sometimes the picture is meant to bright, with a histogram that peaks on the right of the graph – as in this shot. However, it’s important that highlights aren’t clipped, because they shouldn’t all be pure white.

04 Low key
The graph here stacks up hard on the left, but isn’t under-exposed because the peak on the left corresponds with the black background. Shoot so the image is as bright as possible, without clipped highlights.

05 High contrast
Shooting a white building in bright light creates a very-high contrast scene with a histogram that squeezes up at both ends. Detail is going to be lost whatever settings you use. Try a different angle or time of day.

06 Low contrast
Some scenes don’t contain strong shadows or strong highlights and create a histogram that sits neatly in the middle. 
To maximise picture detail, shoot so the histogram sits as far to the right as possible.

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What is a histogram: all your questions answered

I’m new to photography: what is a histogram and what does it do?

A histogram is simply a visual guide to the range of tones or brightness levels in an image. It can be viewed on your camera’s LCD screen alongside an image after you’ve taken it, or displayed during Live View shooting, but either way it shows the same thing: a small graph that displays what tones an image is made up of.

So why do I need it?

Because it shows the spread of brightness levels in a scene, it is the most effective way to judge the exposure of an image. It also gives an indication as to whether you need to make the exposure brighter or darker.

The standard histogram found on digital SLRs measures the brightness on a 256-step scale. The far left of the graph represents 0, or pure black, and the far right of the graph represents 255, or pure white.

The steps between these points are filled with all the possible shades in between, with the centre of the axis being the midtone brightness level.

The height of the histogram at each point along this 256-point scale is an indication of the number of pixels in the shot at that level of brightness. The histogram’s size and shape will change depending on the mix of tones in the scene.

Right, so is there a particular histogram shape – an ideal exposure – that I should be looking for?

Well, the histogram is merely a reflection of what is in front of the camera. If the scene or subject is dark, the histogram should be bunched towards the left, or dark, end of the scale; if the scene or subject is bright, it should be over to the right (if it helps you to remember which is which, right is bright!).

If a scene is mainly made up of midtones – a brown horse in a green field, for instance – then the histogram should be humped roughly in the centre of the scale, but there’s no such thing as a ‘perfect’ histogram.

That said, there are occasions when you might need to make adjustments to get a more accurate histogram, and consequently a more accurate exposure.

Is my camera’s built-in exposure meter not very accurate then?

As we discovered when talking about exposure last issue, your camera won’t always get it right automatically.

Because the built-in light meter is designed to achieve an exposure with an average range of midtones, scenes that are much brighter than midtone (such as bright white snowscapes) will appear grey, or under-exposed; scenes that are much darker than midtone (such as shadow-packed night scenes), will also appear grey, or over-exposed.

The way to rectify this is to use Exposure Compensation to make pictures brighter or darker. To do this, you need to press the ‘+/-’ button and turn your camera’s control wheel. Turn it clockwise to brighten up an image, or anti-clockwise to darken it.

This is where the histogram is most useful, as you can gauge how much brighter or darker to make the image.

Take a shot of a white sandy beach on a bright sunny day, for instance, and you know the histogram should be closer to the right of the scale.

If it’s not, dial-in some positive Exposure Compensation and keep checking the histogram until it is. Make sure you don’t go too far, though, otherwise the histogram will become ‘clipped’.

I’ve heard of the term ‘clipping’ before. Why is this such a bad thing?

If the histogram runs off the left or right of the scale, it means that part of that picture information is being lost. If the histogram is clipped on the left, the darkest shadows are said to be ‘crushed’, as they will be pure 
black, with no detail.

This is fine if you want, say, shadows or the night sky to be pure black, but even with shadows and night skies you usually want some texture or detail.

When the histogram is clipped on the right, the highlights are said to be ‘blown’, and will render as pure white, again without any texture or detail.

That said, there are times when capturing clipped highlights will be unavoidable. Specular highlights – those bright, reflected spots of light that appear on shiny objects – are supposed to be bright white, and don’t actually contain any important texture or detail.

So I should always make sure there’s space between the right side of the histogram and the edge of the scale?

Yes, if you can, but don’t leave too much of a gap. You should always try and expose digital images so that they’re as bright as possible, but not clipped, because for technical reasons the sensor can record more tonal information towards the bright end of the histogram.

Is there ever a chance that the histogram has got it wrong?

No, it’s simply a representation of tonal range. On some cameras, the graph is small, so it can be hard to judge whether the edges of the histogram are at the extremes, but the highlight alert feature will indicate any over-exposed areas.

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In the latest of our photography cheat sheet series we’ve compiled this handy guide for explaining how this exposure chart represents the range of tones withing your scene. Simply drag and drop this chart on to your desktop and keep it for a handy reference the next time you find yourself confused as to how to read a histogram.

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But there’s no ‘perfect’ histogram. Each landscape scene you shoot is made up of a different blend of tones, and the shape of the histogram will reflect this.

A midtone landscape scene, such 
as a church surrounded by rolling green hills, is likely to show a histogram that’s humped around the middle 
of the scale, whereas trees silhouetted against a bright dawn sky would show a histogram that’s flat through the middle, but which peaks on the left and right (as the scene consists of just dark and light tones).

If the histogram goes beyond the edges of the scale, the picture information is ‘clipped’, and you’ll start to lose detail.

Below we’ve provided several typical landscape scenes that we might shoot here in the UK and how to interpret the reading on your histogram.

Example histograms… and what they mean

The built-in exposure meter has attempted to bring this naturally bright scene closer to the value of midtone grey, so the histogram is bang in the middle

By dialling-in positive Exposure Compensation, the brightness of the scene is restored, and the histogram shifts to the right (bright) end of the scale

Here, the exposure meter has again tried to bring this naturally dark scene closer to the value of midtone grey, so what should be dark is too bright and washed out

By dialling-in negative Exposure Compensation, the scene is restored to its natural darkness, and the histogram shifts closer to the left (dark) end of the scale

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Getting your head around exposure can be daunting, so let’s start with the basics. When taking photographs, an image is recorded by light reaching your camera’s sensor. You need a certain amount of light to expose the scene correctly; too little and the image will be too dark, or under-exposed; too much and it will be too bright, or over-exposed. The amount of light reaching your sensor is controlled by three key components: aperture, shutter speed and ISO.

The aperture controls how much light is allowed through the lens – a wide aperture lets in more light, a narrow aperture less. The shutter speed determines the length of time the shutter remains open. Aperture and shutter speed work in unison to expose the image correctly, and if you adjust one you have to adjust the other: if, for example, you increase the shutter speed (therefore decreasing the length of time that light hits the sensor), then you have to use a wider aperture to let in the same amount of light to expose the shot correctly.

The other variable that affects exposure is ISO. The ISO setting affects the sensor’s sensitivity to light. The lower the ISO, the more light is required to expose the image.

Understanding exposure

To determine the aperture and shutter speed required to expose a shot correctly at a given ISO, your camera measures the amount of light reflected back from the scene using a built-in light meter. The key word here is ‘reflected’. Camera meters assume that the scene you want to photograph contains a full range of tones, and tries to expose it accordingly. This means that certain lighting conditions can confuse them – dark scenes can be over-exposed and bright scenes under-exposed – because the meter will try to expose the light or dark areas as midtones.

Dark scenes

A built-in light meter will try to produce an exposure made up of average midtones. A dark scene that’s been exposed so it’s dominated by midtones will therefore look over-exposed.

Mid-tone scenes

Scenes containing a relatively even mix of shadows, midtones and highlights, such as in the photo above, won’t give your camera’s metering system too many problems.

Bright scenes

As with dark scenes, because the built-in light meter will try to render any scene as an average midtone, very bright subjects like snow will end up looking a bit grey, rather than pure white.

How to read a histogram

You can call up the histogram on your camera’s LCD to judge the tonal distribution in a photo. The horizontal axis shows pixel brightness, ranging from pure black on the left to pure white on the right. The vertical axis shows the number of pixels at a particular brightness level.

Under-exposed

Not enough light has reached the sensor, caused by either too fast a shutter speed, too narrow an aperture, or both. This shot’s histogram is pushed all the way to the left, a situation known as ‘clipping’. If printed, the clipped areas will appear completely black, and all shadow detail will be lost.

Correctly exposed

The histogram should correspond with the tones of your scene; a dark scene should have a histogram with a bell shape on the left; a light scene should have a histogram with a bell shape on the right. For daylight scenes, expose the scene as far to the right as possible without clipping the highlights.

Over-exposed

Too much light has reached the sensor. Notice how the histogram reveals that the tonal distribution is pushed all the way to the right, and that the highlights are clipped. In the printed photo, areas of the scene that should reveal detail will appear as pure white, with no detail.

Metering modes

Multi-zone metering

This is also known as matrix, evaluative, multi-segment and pattern metering, depending on the camera you have, but all serve the same purpose. Multi-zone metering is generally the default setting on your camera when you first switch it on. In this mode, the camera divides the scene into sections and takes a reading from each section to determine an overall reading for the whole scene.

Centre-weighted metering

Centre-weighted metering also takes a reading from the whole scene, but concentrates mainly on the central 60% of the frame. It’s handy for portraits, especially when the model is in the centre of the frame. Centre-weighted metering can easily be fooled by very bright or very dark areas, but it can be easier to predict when you need to adjust the exposure than it is with the multi-zone metering mode.

Spot metering

This is the most accurate metering mode because it enables you to take a reading from a small, precise area of a scene. However, it can take a bit of practice to be able to judge precisely what constitutes a suitable midtone. To simplify matters, some photographers meter off a so-called ‘grey card’ placed in the same light source as the subject. You can pick one of these up from Jessops for around £6.

Back to: Explore your SLR

Forward to: Understanding aperture

Your camera histogram is a much more accurate way of judging whether a shot needs a bit more, or less, exposure. But it’s not just used for judging exposure: the shape and position of the histogram’s graph can also tell you about the contrast of the lighting in a scene. Below we’ve tackled some of the frequently asked questions about histograms to get you started using these helpful graphs to avoid poor exposures.

What is a histogram?

What is a histogram and why do I need to use one?

In short, the histogram is a type of graph that you can call up on your digital camera’s rear LCD in order to judge the tonal distribution of the images you take. The histogram offers a way of plotting the exposure of a digital image; by looking at the graph, you can see whether a shot is too bright or too dark.

The histogram’s horizontal axis shows pixel brightness, ranging from pure black on the left to pure white on the right, with the full range of mid-tones in between. Its vertical axis shows the number of pixels at a particular brightness level.

How do I view the camera histogram?

You usually look at the histogram after you have taken the picture, when reviewing the shot on screen. Look at your instruction manual to find out how you get this graph shown on the LCD (and to check your digital camera has this facility).

On Canon DSLRs, for instance, press the Play button, then use the Info or Disp button to call up this display option. On Nikon DSLRs you can use the up arrow on the joypad during playback to toggle through the display options.

What is a histogram showing me exactly?

The graph shows the brightness of all the pixels in the image. The brightness is plotted along the x-axis, or bottom line, and the number of pixels is plotted along the y-axis, or vertical line.

To keep things simple, the numbers and units are not shown on the graph. All you need to remember about your histogram is that the left-hand side shows the darkest tones, the right-hand side shows the brightest tones and the central part of the graph shows the midtones.

What shape should I aim for on my histogram?

The shape of the graph is highly dependent on what you are taking pictures of. If you are shooting snowy scenes, for example, the histogram should peak towards the right side of the graph because of all the bright tones.

For normal subjects, with an average distribution of different tones, the ideal graph rises gradually from the far left outside, peaks somewhere in the middle, then falls away gradually until it has fallen to zero at the far right-hand side, as you can see in the histogram inset below.

As evidenced in this shot of a red kite, the histogram should correspond with the tones of your scene; a dark scene should have a histogram with a bell shape on the left, a light scene should have a histogram with a bell shape on the right. For daylight scenes, try to expose the scene as far to the right of the graph as possible without clipping the highlights.

How can I tell if the exposure is wrong in my shot?

The telltale sign is if the graph is stacked up to the left or the right on your histogram. If there is a peak at the extreme left-hand side of the graph, this shows that there are lots of pixels that are recording as the darkest value black available. This suggests that your shot is probably under-exposed.

In this photo of a red kite in flight, too little light has reached the sensor, caused either by too fast a shutter speed or the aperture not being wide enough. This shot’s histogram is pushed all the way to the left, a situation known as ‘clipping’. If printed, the clipped areas will appear black, and all shadow detail will be lost.

Similarly, if the histogram looks as if it’s pushed hard to the right of the graph area, it’s likely that the shot is over-exposed.

This shot of the same red kite is overexposed, as too much light has reached the sensor. Notice how the histogram reveals that the tonal distribution is pushed all the way to the right, and that the highlights are ‘clipped’. In the printed shot, areas of the scene that should reveal detail will appear pure white, with no detail.

How do I change the shape of the graph?

The simplest way to do this is to take another picture of the same subject using a different amount of exposure. Take the shot with less light and the subsequent graph will have shifted to the left.

Shoot with more light, and the graph will move to the right. Use the +/- exposure compensation control to do this; a plus value moves the histogram to the right, a negative value shifts the graph left.

What if the histogram is stacked to the left and the right?

This shows you that the scene you are photographing has both very bright highlights and very dark shadows. It is a sign of a high-contrast scene where the camera is going to find it impossible to find an exposure that captures detail and tone in all parts of the scene.

Sometimes, you can get round the problem by changing the lighting (so you are not shooting into the sun, for instance, or by coming back later in the day), or by using an ND grad filter to reduce the brightness of the sky. If you have to shoot, it is usually best to adjust the exposure so the graph moves to the left, because it is easier to rescue the shadows than the highlights at the editing stage.

Is the histogram always right?

It’s the picture that counts, not the histogram – so look at the image too. The graph itself is based on the tones that would be captured by a JPEG, so if you are shooting raw your image will capture more detail in the shadows and highlights than the graph actually shows.